1. Field of the Invention
The present invention relates to a sheet feeding apparatus and image forming apparatus. Specifically, the present invention relates to a sheet feeding apparatus and image forming apparatus configured to separately feed sheets by blowing air to the sheets.
2. Description of the Related Art
In the related art, an image forming apparatus such as a printer, a copying machine has a sheet feeding apparatus that feeds, one by one, sheets carried on a tray holding a plurality of sheets. As such a sheet feeding apparatus, there is a sheet feeding apparatus of an air feeding system to blow up a plurality of sheets by blowing air to an end portion of a sheet bundle held by a tray, adsorb the sheets to an adsorption feeding belt arranged upward and feed the sheets one by one (see U.S. Patent Application Publication No. 2005/0206068 A1 and U.S. Patent Application Publication No. 2009/0267288 A1).
This air-feeding-system sheet feeding apparatus loosens sheets by blowing air to a leading-end-side edge portion of a sheet bundle on a tray and blowing up the sheets, and adsorbs the topmost sheet of the blown sheets to an adsorption conveyance belt by negative pressure. Further, by rotating the adsorption conveyance belt to which the sheet is adsorbed, it is possible to feed sheets one by one to the downstream side. By this means, the sheets are separately fed one by one to an image forming portion. This air-feeding-system sheet feeding apparatus has a higher resistance than a sheet feeding apparatus of a general friction separation system. Therefore, this air-feeding-system sheet feeding apparatus is often used in a field of simple bookbinding (e.g. light printing of a booklet or catalogue) using an image forming apparatus of an electrophotographic system, called POD (Print On Demand).
In recent years, it is demanded by users to increase productivity (i.e. the number of formed images per unit time) in an image forming apparatus. Especially, in the above-noted field of POD, it is necessary to perform light printing in volume, and therefore a sheet feeding apparatus of increased productivity is demanded. Generally, to increase productivity, it is necessary to increase the number of fed sheets per unit time in a sheet feeding apparatus. Therefore, in an air-feeding-system sheet feeding apparatus in the related art, there is a sheet feeding apparatus that, after separating sheets one by one, overlaps part of the next sheets on the separated sheets and conveys these.
However, in such a sheet feeding apparatus, to increase the number of fed sheets, it is necessary to not only overlap and convey sheets but also speed up the feeding speed of sheets to be fed. Here, to speed up the sheet feeding, it is necessary to blow up sheets at higher speed and speed up the conveyance speed of an adsorption conveyance belt.
Here, to blow up sheets at higher speed, it is necessary to speed up (or increase) the wind speed (or air volume) of air to be blown. However, if the wind speed (or air volume) is speeded up (or increased), regarding thin (i.e. the basis weight is small) sheets, these sheets are blown up all at once and cannot be loosened reliably. By this means, there arises a problem that a plurality of sheets is adsorbed to an adsorption conveyance belt and multi-fed.
Also, if the conveyance speed of an adsorption conveyance belt is excessively speeded up, regarding thick (i.e. the basis weight is large) sheets, there is a case where the sheets are fed without being reliably adsorbed to the belt due to fictitious force. That is, when sheets are sequentially fed and a position of the topmost sheet becomes low, there is a case where sheet adsorption starts by the time the position of the topmost sheet rises to a height at which adsorption is reliably performed. In this case, a sheet feeding delay is caused and a sheet jam may be caused. Thus, depending on a sheet basis weight, when sheets are fed in an overlapping manner, the overlapped parts may vary and a sheet multi-feed or jam may be caused.
The present invention is made in view of the above problems and has an object of providing a sheet feeding apparatus and image forming apparatus that can reduce the variability of overlapped parts when feeding sheets in an overlapping manner.